In recent years, a printed-wiring board having a circuit pattern formed on an insulating substrate, such as a plastic substrate, a ceramic substrate or a metallic substrate coated with plastic, has been developed. Means to configure an electronic circuit by soldering electronic components, such as an IC element, a semiconductor chip, a resistor and a condenser, on the circuit pattern has been widely adopted.
In this case, for the purpose of bonding the lead terminals of the electronic components at the stated parts of the circuit pattern, the procedure which comprises a step of preparatorily forming a thin layer of solder on the surface of a conductive circuit electrode on the substrate, printing a solder paste or a flux thereon, positioning and loading the electronic components thereon and then reflowing the thin layer of solder alone or the thin layer of solder in conjunction with the solder paste, thereby bonding the layer by soldering is generally followed.
Recently, the electronic circuit boards have been in need of satisfying the trend of fine pitch with a view to miniaturizing electronic products. Fine pitch parts, such as, for example, the LSI of the QFP (quad flat package) type and the CSP (chip size package) each having a pitch of 0.3 mm, and the FC (flip chip) having a pitch of 0.15 mm, have come to be mounted on the electronic circuit boards in numerous quantities. The electronic circuit boards, therefore, are required to have a fine solder circuit pattern which conforms to the fine pitch.
The formation of a solder circuit with a solder film on the printed-wiring board is being accomplished by the plating method, the hot air leveler (HAL) method, or the method which comprises a step of printing the paste of solder powder and a step of reflowing the printed paste. The method for producing a solder circuit by the plating technique, however, incurs difficulty in adding to the thickness of the solder layer, and the HAL method and the method resorting to the printing of a solder paste encounter difficulty in coping with the fine pitch pattern.
As a way of forming a solder circuit without requiring such a troublesome operation as aligning a circuit pattern, a method which comprises causing a stickiness-imparting compound to react with the surface of a conductive circuit electrode of a printed-wiring board, thereby imparting stickiness to the surface, depositing solder powder on the sticking part consequently formed and then heating the printed-wiring board, thereby melting the solder and forming a solider circuit has been disclosed (refer, for example, to JP-A HEI 7-7244).
By the method disclosed in JP-A HEI 7-7244, it has been made possible to form a fine solder circuit pattern by a simple operation and provide a highly reliable circuit board. Since this method effects dry deposition of the solder powder on the circuit board, it entails such problems as suffering the powder to be electrostatically deposited on an extraneous portion, inducing drift of the powder, interfering the fine spacing of a circuit board and preventing the powder from being efficiently utilized. These problems gain in conspicuousness particularly when the solder powder to be used has a very fine size.
This invention is aimed at solving these problems and, in the method which comprises causing a stickiness-imparting compound to react with the surface of a conductive circuit electrode of a printed-wiring board, thereby imparting stickiness to the surface, depositing solder powder on the sticking part consequently formed and then heating the printed-wiring board, thereby melting the solder and forming a solder circuit as disclosed in JP-A HEI 7-7244, at providing a method for the production of an electronic circuit board capable of realizing a still finer circuit pattern, an electronic circuit board provided with a very fine circuit pattern and endowed with high reliability, and a circuit board having mounted therein electronic components and enabled to materialize high reliability and high mounting density.